This invention concerns a method and apparatus for testing workpieces with ultrasonic energy and more specifically concerns a method and apparatus for compensating for changes of the sound velocity of the coupling liquid as a function of temperature.
When testing workpieces with ultrasonic energy the energy is cyclically transmitted from an electroacoustic transducer to the workpiece via a coupling liquid, mostly water. The ultrasonic energy transmitted is reflected at the workpiece surface, at an acoustic discontinuity within the workpiece and also at the rear surface of the workpiece. The reflected energy is transmitted back to the transmitting transducer or to a separate receiving transducer and is converted by the transducer to an electrical signal which, in turn, is sent to an evaluation circuit. The transit time of the ultrasonic pulse while transversing the coupling liquid is an important parameter for determining the geometry of the workpiece under test.
In ultrasonic test systems the workpiece is frequently moved at a substantially high speed relative to the transducer. Relative motion may comprise one or more transducers being rotated about a workpiece, a linear motion of transducers parallel to the workpiece surface, or a combined rotational and linear motion, i.e. helical scan. In a typical arrangement for testing tubes or cylindrical workpieces ultrasonic transducers are rotated about the workpiece while the workpiece is translated in the axial direction. Such test arrangements are used to determine the inner and outer diameters and the eccentricity, particularly the degree of out-of-roundness, by measuring the transit time of the ultrasonic signal traversing the coupling medium and the wall of such a tubular workpiece.
When it is desired to use such rotational arrangements for determining with a high degree of precision the out-of-roundness and the outer diameter of a tubing or cylinder an important requirement is that the acoustic velocity of the coupling liquid remain constant during the measuring period. However, given the changes arising from the working environment such constant conditions are not possible. Therefore, it is required that the influence of the temperature, i.e. the temperature responsive change of the sound velocity of the coupling liquid, be compensated. For instance, using water, the velocity of sound changes at the rate of 2.5 meter per second per degree K. Assuming a nominal sound velocity of 1480 meter per second, room temperature, and a liquid coupling path of 15 mm, a measuring error of 0.051 mm per degree K. occurs for each coupling path, however is must be kept in mind that the total ultrasonic signal path distance is 30 mm, i.e. 15 mm gap distance which must be traversed by the acoustic pulse in both directions. If two diametrically opposite measuring paths are used as is commonly the case, the measuring error amounts to approximately 0.1 mm per degree K. Systems currently in use permit, however, a measuring accuracy of a few thousandth mm.
For overcoming the temperature induced variation of the sound velocity in the coupling path, two solutions are known.
In the first arrangement regulating means are used to maintain the temperature of the coupling liquid constant to the extent of a few tenth degree K. This method is slow and it is necessary to adapt the system to the prevailing test conditions. A temperature sensor must be provided in the liquid which controls the heating or cooling of the liquid. The temperature of the coupling liquid is controlled from sources independent of the measuring arrangement.
In the second arrangement the changes of sound velocity are compensated. For this purpose an auxiliary measuring distance comprising a transducer and reflector, independent of the actual measuring path, is provided within the coupling liquid and the transit time of ultrasonic energy traversing this path is measured on a continuous basis. In order to avoid determining the precise sound velocity only the temperature responsive transit time change is considered and, hence, a distance compensation distance is required which is adjusted by means of very complicated and highly precise mechanical means to provide an auxiliary measuring distance exactly equal to the sum of both coupling paths between the respective transducers and the workpiece surface. The disadvantage of this arrangement resides in the fact that there usually is little space for such auxiliary measuring means between the rotating workpiece and the transducer disposed in close proximity thereto.
Moreover, the auxiliary arrangement requires a separate electronic circuit, see H. E. Gundtoft et al, Materialpruefung 19 (1977) No. 9, pp. 385-388.
Compensation by the use of separate temperature sensors is also disadvantageous since further means are needed within the coupling liquid in close proximity to the measuring path and compansation requires the use of a function table (i.e. electronic memory) which provides the sound velocity as a function of the temperature measured.
Another known system used primarily in rotating test systems utilizes auxiliary reflectors disposed in the sonic energy path between the measuring transducers and the rotating workpiece, and for shortening the actual measuring path such reflectors are located very close to the workpiece surface. The distance of these reflectors to the transducer is either predetermined or is adjustably fixed.
The functional significance of the auxiliary reflector is not considered further herein inasmuch as this is not important for this invention. If the auxiliary reflectors are not present in a system, they can be installed in the sonic energy path without any problem since no electrical connections are necessary. With regard to such auxiliary reflectors reference is made to Gundtoft supra and German OS No. 21 48 976.